We did Alaska last July on Holland America. We have found the solution to be layers - we did this for Norway and the Arctic circle as well and remained dry and cosy. Hope other people can tell you about the activities but the cruise is so wonderful you may not have much time! Watching glaciers would be serious competition for any other activity. Enjoy - we would go back.
We did Alaska on Norwegian Sun in June 2006 and it was a wonderful cruise. The ship's climate is good so there is no need for fleeces etc. unless you are going on deck (it can be cool when the ship is moving). Layers, as advised by Griffiths, is very sound advice. All the ports of call were warmer than we expected and the only time we needed fleeces plus anoraks was when were on the whale watch - which is superb. One tip is to do the NCL trip in a small boat up the Tracey Arm glacier - people that we spoke to had a brilliant time and saw lots of whales, seals etc. and aslo got close up to the glacier. Unfortunately the captain had to turn the Norwegian Sun around before we reached the glacier for safety reasons (although this was a brilliant sight to see). The NS is a lovely ship and the entertainment - which we hadn't expected to enjoy - was first class most of the time. Announcements for bingo are irritating but this was only a small niggle. Sailing from Vancouver was also very memorable as the weather was lovely and we had a convoy of jet skiers and power boats to accomapny us! Enjoy, we certainly did.
We have baeen on a few Alaska cruises, twice in August. It will be warm inside the ship but as you will be spending a lot of time on deck soaking up the atmosphere and scenery, layers are the best solution as already stated. Alaska can be warm, cold, dry and wet in August. Ketchikan is one of the the wettest towns in the US so go prepared. I don't have an anorak but always pack a lightweight waterproof kagoul. That with a light fleece jacket is fine to throw on when needed. When kayaking, I wear shorts, t shirt, fleece jacket, sports sandals and the waterproof top and safety vest supplied. On deck and for most excursions that do not include hiking or water sports, trainers are fine. I found a sweat shirt/jeans combo fine for the Tracey Arm trip with the kagoul as a just in case. I always take a small rucksack and stuff anything I think we might need for the day into it. Shorts, jeans, t's, sweatshirt, a fleece,trainers, deck shoes, Teva sport sandals, all are casual and useful. Think UK spring and add the layers for warmth and shorts for if it's very warm.